1. Field of the Invention
The present invention relates to an image pickup apparatus that performs an image pickup operation while controlling exposure according to the brightness of an object, a method of controlling the image pickup apparatus, and a storage medium.
2. Description of the Related Art
In general, an image pickup apparatus using a solid-state image pickup device, such as a CCD or a CMOS, controls the exposure amount of the image pickup device by operating a diaphragm or adjusting shutter speed, to thereby control the amount of exposure to the image pickup device. For example, in the case of shooting a bright scene by the image pickup apparatus, it is possible to increase the shutter speed to thereby adjust the brightness of an object to be recorded in a picked-up image to an appropriate brightness. On the other hand, in the case of shooting a dark scene by the image pickup apparatus, it is possible to reduce the shutter speed to thereby adjust the brightness of an object to be recorded in a picked-up image to an appropriate brightness. In short, the image pickup apparatus is capable of suppressing whiteout or blackout by controlling the amount of exposure to the image pickup device.
There has conventionally been proposed an image pickup apparatus that employs a method in which whiteout or blackout is detected based on the brightness frequency distribution of brightness over an entire screen determined from a picked-up image signal, and the amount of exposure is controlled according the detected brightness frequency distribution to thereby suppress whiteout or blackout (see e.g. Japanese Patent No. 4306750).
However, even though the above-mentioned image pickup apparatus is capable of detecting whiteout and blackout, it is difficult for the image pickup apparatus to determine whether whiteout areas or blackout areas are scattered or concentrated.
This reason will be explained with reference to FIGS. 12A to 12D. FIG. 12A is a view illustrating a picked-up image in which whiteout has occurred in a concentrated manner by a void portion. FIG. 12B is a view illustrating a picked-up image in which a plurality of point light sources existing in a scatter manner has caused whiteout areas in a scattered manner. FIG. 12C is a diagram showing brightness frequency distribution (histogram) of the FIG. 12A picked-up image, and FIG. 12D is a diagram showing brightness frequency distribution of the FIG. 12B picked-up image.
In FIG. 12A, the whiteout areas are concentrated, while in FIG. 12B, the whiteout areas are scattered. However, as shown in FIGS. 12C and 12D, the brightness frequency distributions have the same shape because the picked-up images in FIGS. 12A and 12B have the same total whiteout area.
In the case shown in FIG. 12A, since the whiteout areas are concentrated, whiteout is visually conspicuous. There is a possibility that gradation is actually lost in the whiteout areas, and hence there is a possibility that void gradation can be reproduced by performing exposure control such that the amount of exposure is reduced.
On the other hand, in the case shown in FIG. 12B, since the whiteout areas are scattered, whiteout is not visually conspicuous. For this reason, reduction of the exposure amount makes noise conspicuous in the picked-up image. Therefore, exposure control should not be performed such that the exposure amount is reduced. In other words, it is desirable that the exposure control is performed while targeting only visually conspicuous whiteout areas.
However, no conventional image pickup apparatus is capable of discriminating between the two scenes in FIGS. 12A and 12B and performing exposure control differently according to the result of the discrimination.